It is known that a gear tooth flank with an improved surface roughness (lower surface roughness) has better durability and transmission efficiency, and makes little noise.
The prevailing method for finishing a tooth flank of a high-hardness gear is a highly efficient gear grinding method in which opposite tooth flanks of a work gear are finished simultaneously while the work gear is being forcibly rotated synchronously with flank surfaces of a threaded grinding wheel rotating at a high speed. In the gear grinding method, a flank surface of a grinding wheel (grinding wheel tooth flank) engages a tooth flank of a work gear without any gap therebetween (zero backlash). Accordingly, cutting speed of the grinding wheel can be controlled, whereas machining pressure on the work surface is difficult to be controlled. As a result, the pressure on the work point on the tooth flank fluctuates depending on machining conditions or machining circumstances (e.g., sharpness and clogging of the grinding wheel, and machining allowance), which sometimes generates a high pressure beyond the limit of the work gear, possibly leading to a grinding burn and a crack on the tooth flank of the work gear, or damage to the grinding wheel.
In a conventional gear grinding method, even if a finishing grinding wheel (soft or fine polishing powder) is used to lower the surface roughness of the tooth flank, because the method is performed under a zero backlash condition, which makes it difficult to control the machining pressure, the grinding wheel may be damaged, and thus the method has limitations in lowering the surface roughness. That is, for lowering the surface roughness, it is desired to control the machining pressure to achieve an appropriate machining speed.
Other tooth flank finishing methods include the gear honing method. However, as in the Patent Literature 1, in the method, tooth flanks of a grinding wheel engage a gear under zero backlashes to simultaneously finish opposite tooth flanks of the gear. This makes it difficult to finely control machining pressure between the tooth flanks of the grinding wheel and the tooth flanks of the work gear.